Even though my wife is right next to me... My wife's best friend (a little devil) whispered and seduced during the trip, two days of a hot spring affair with a two-day night

I have a secret that I can't tell my wife's best friend, Sumire, and my partner. From the day they happened to meet him at a bar, they have been involved in an affair as they were swayed. My wife is important, but it's an unstoppable and convenient relationship... The relationship changed completely when the four of them, a couple, decided to go on a hot spring trip. I was skeptical about Sumire saying that she should have sex while on the trip, but on the day Sumire's husband developed a high fever, and the three of them ended up going on a two-day trip together...